1. Field of the Invention
The present invention relates to a silicon nitride ceramic heater having an excellent heating function and being superior in durability.
2. Prior Art
An alumina ceramic heater with a heating element of high-melting point metal embedded in an alumina ceramic sintered body is generally used as a ceramic heater. However, since alumina is inferior in thermal shock resistance and high temperature strength, a heater intended for use at high temperatures with W, WC, TiN, etc. embedded as a heating element in a silicon nitride ceramic sintered body has been used (refer to Japanese Patent Publication Nos. 62-19034 and 62-59858).
Since a silicon nitride ceramic material is superior in heat resistance, high temperature strength and electrical insulation, and has small heat capacity, it is excellent for use in heaters. For this reason, the above-mentioned silicon nitride ceramic heater is superior in quick heating function, thermal shock resistance and high temperature stability, and is widely used for glow plugs of automobile engines, vaporizers of kerosene fan heaters and other home appliances, electronic devices and industrial machines.
However, the silicon nitride ceramic material has a low thermal expansion coefficient of 3.1.times.10.sup.-6 /.degree. C. On the other hand, heating elements individually made of W, WC and TiN for example have high thermal expansion coefficients of 4.6.times.10.sup.-6 /.degree. C., 5.1.times.10.sup.-6 /.degree. C. and 9.4.times.10.sup.-6 /.degree. C., respectively. Because of this difference, when such a ceramic heater is used at high temperatures and subjected to heating-cooling cycles, the heating element of the heater may become cracked or the electrical resistance thereof may be changed owing to the fatigue caused by the thermal stress generated from the difference in thermal expansion coefficients.
To solve these problems, the operation conditions of the heater must be restricted depending on the thermal expansion coefficient of the heating element, thus causing inconvenience.